Journal of geography and geology/ jurnal geografi dan geologi

1. Journal of Geography and Geology; Vol. 4,No. 4; 2012
ISSN1916-9779 E-ISSN1916-9787
Published by Canadian Centerof Science andEducation
Rill Hydraulics - An Experimental Study on Gully Basin in Lateritic
Upland of Paschim Medinipur, West Bengal, India
Abstract
This paper reports the results of field investigations aimed to establish relationship between
morphologic and hydraulic characteristics of rills. A rill network within a small Rangamati Gully Basin,
on the bank of Kansai atPaschim Medinipur,in West Bengal,India of 256 m2 was mapped.
Experimental basin is exposedto natural rainfall of varying intensity and characterised with an
average of 25-35% slope gradient. The depth, length,gradient,width, runoff ontributing areas of all
the 33 rills within the basin were recorded and thoroughly mapped. Present study incorporates close
monitoring of runoff and velocity along the channels during a storm on
31.08.2010 at 15 minutes interval using dye tracer technique.The velocity along each rill was linked
with gradient, width-depth ratio and Manning’s roughness coefficient.Analysis shows that flow
velocity is not directly controlled by rill gradient (R2=0.15),but is influencedby hydraulic roughness
coefficient(R2=0.53).
Finally, the relationship between Reynolds number and Froude number reveals that sub-critical
turbulent flow is responsible for rilling process. No significant relationship between W/D ratio and
channel roughness (n) can be established (R2=0.083)by the present study.
Keywords: rills morphology, rills hydraulics, roughness coefficient,velocity
1. Introduction
Rills are an integral part of the runoff system of many environments.Rills are small, ephemeral
concentrated flow paths which function as both sediment source and sediment delivery systems on
upland area. Hydraulic and geomorphic factors of rills are important for understanding the
mechanism of rills formation, and spatial as well as temporal variation of geometriccharacteristics.
The geomorphic factors influencing the location and geometry of rills are also related to the
variability of soil properties and the landscape (Mancilla et al., 2005). The
geomorphic factors reveal that rill initiations are affected by the flow, the roughness of the soil
surface,the slope gradient, and the erodibility of the soil (Van et al., 1983; Foster et al., 1984; Bryan et
al., 1989; Gilley et al.,1990; Slattery et al., 1992; Obiechefuet al., 1994; Favis-Mortlock et al., 2000).
On the other hand, the hydraulic factors consider flow-related parameters. Flow depths in rills are
typically of the order of a centimeter or less on steeperslopes. These conditions constitute a very
different hydraulic environment that typically found in channels of streams and rivers (Nearing et al.,
1997). No widely acceptedformula exists to representthe relationship of discharge, flow velocity
and gradient. The flow velocities in rills were developed in loose, non-layeredmaterials dependingon
flow discharge, while bed slope and hydraulic roughness are irrelevant (Nearing et al., 1997; Govers,
1992; Nearing et al., 1999). Gimenez and Govers (2001)
also showed the slope independence of rill flow velocity by identifying feedback mechanism
betweenrill bed roughness and flow hydraulics. On the contrary, the mean flow velocities appeared
to dependon both flow

2. discharge and slope of the rill (Foster et al., 1984; Abrahams & Li, 1996). Yet, for situations where a
rill is able to adjust freelyits geometry to the flow condition impact of slope on flow velocity
becomesinsignificant(Takken & Govers, 1998). Later, Gimenezand Govers (2000) showed that
runoff velocity dependson slope when the flow is not capable of eroding the rill channel.
Width-discharge relationship may be extendedto channels as small as rills and gullies(Nachtergaele
et al., 2002).
Torri et al. (2006) established width-discharge relationship for rills and ephemeralgullies on arable
land that shows variable discharge exponentdependingon rill width. Savenije (2003) stated that the
power relationship betweendischarge and channel width is theoretically derivable for the case of
bankfull discharge and may be applied to rills and gullies at episodic bankfull discharge condition.
Various studies have investigated of the hydraulic roughness of rills. Chow (1959) and Savat (1980)
studied the flume experimentson rill; the results indicate a progressive decrease in hydraulic
roughness (n) with increased Reynolds number (Rn). In a case,if depth of water is less than the size
of the physical roughness coefficient,thenMannings Roughnesscoefficient increasesin
correspondence with increase in Rn values (Govers, 1992;Abrahams et al., 1986; Abrahams &
Parsons, 1994; Gilley et al., 1992; Prosser & Dietrich,1995). The spatial variability of rilling process
depends on flow character controlled by channel roughness and expressedas
Reynolds number (Rn) and Froude number (Nearing et al., 1997).
However,the hydraulic considerations must involve flow velocity, which is a keyfactor of the energy
neededfor rill development and soil erosion (Mancilla et al., 2005). Flow velocity has beendirectly
related to rill formation and development (Lei et al., 1998) and leads to variable rates of soil loss
(Nearing et al., 1999; Mancilla, 2001).
Rill flow velocity has beenalso related to the transport capacity of the flow regime and thus to
sediment delivery (Lei et al., 1998; Mancilla, 2001). In particular, rill morphology at a giventime is
associated to hydraulic roughness features, rills width, and rills depth which are functions of the
eroding material, runoff rate, and prior rill structure. Erosion creates the morphology of a rill channel
and in turn the rill structure influencesthe erosion (Nearing et al., 1997).
Present researchfocused on rill morphology and its hydraulic characters in lateritic upland area as
limited researchhas beendone on this specific field. Specificobjectives of this paper were: (1) to
establish relative importance of eitherroughness coefficient,slope gradient and width-depth ratio to
control flow velocity and (2) to identify the hydraulic regimes through relationship of Froude
numbers and Reynolds numbers in the present situation of rill-gully development.
2. Study Area
The study area covers an area of about 256 sq m and is bounded by 22°24'42.0"to22°24'43.2"N
latitude and 87°17'48.1" to 87°17'48.09" Elongitude (Figure 1). Total network of the basin is
composed of 33 rill links and are marked with certain identity. These rills are classified following
Strahler (1964) ordering scheme (Figure 2).
A tropical, monsoonal climate prevails in, with mean annual temperature of around 28.4°C, and
average summer (May) and minimum winter (December)temperatures of 40.9°C and 7.5°C
respectively.Mean annual rainfall is about 1850 mm. Rainfall distribution is irregular, experiencing
high-intensity rainstorms during June to September.Erosive potential of rain is very high where the
rainfall erosivity factor (R) varies between1200 and 1500 MJ mm ha-1h-1year-1.The soil is lateritic in
type having sandy loamy characters with a weak geological formation consisting of Tertiary to early
Quaternary unconsolidated sandstones (Shit & Maiti, 2008).
One of the main characteristics of the area is the dissection of the landscape by a dense and deep
network of rills and gullies. Inter-gully areas are usually undulating and rolling. The average slope of

3. this area is between25% and 35%. The most frequent landforms are complex slopes and gullies. The
rills are characterizedby vertical sidewalls of 7-13 cmand are 90 cm wide on an average. Rill has a
high degree of lateral as well as headretreat.
The nick points, developed at the vertical head near the source,sometimes shows the tension cracks
for toppling, where centre of gravity overlies the centre of mass (Shit & Maiti, 2008). Sediment,
mobilized from the walls and vertical heads,is usually removedby flowing water during high
intensity rainstorms. In other cases, the sediments are deposited on the base of walls or on the gully
bottom, which may lead to some degree of stabilization.
3. Materialsand Method
3.1 Monitoring Networks
33 rills under a gully basin are monitored to study morphological and hydrological attributes like rill
length (RL),gradient (S), width (W), depth (D), runoff contributing area (RCA), velocity (v), and
discharge (Q) with appropriate field techniques(Figure 3). The gradients (S) of the channels were
measured with clinometers.The junctions were mapped and attributed with identity for recording
hydrological and morphological parameters of the links connecting those nodal points. Longitudinal
profile and gully cross-sections were monitored after peak rain intensity. Runoff contributing area
for eachtributary rill is identified from detailed contour map prepared at 0.1 m interval. Width, depth
and lengths are monitored at each junction with pegging techniques.
3.2 Runoffand Velocity Measurements
The experimental methodology involved collection of surface runoff by ‘V’ flume using bottle at each
junction points of the gully basin. Rainfall was measured by self recoding rain gauge during the
monitoring period following Jehangir and Zokaib (2000) and Zokaib et al. (2005). The average flow
velocity was measured in individual rill using a dye-tracing technique following Abrahams et al.
(1986), Gilley et al. (1990) and Govers (1991, 1992). Potassium permanganate solution was used as a
dye,where a small amount of which was injected at the source of the links. Flow velocities were then
measured by recording the travel time of the dye cloud over the distance of link length.The average
travels time was taken as the mean of five measurements(Finkner &Gilley, 1988).
In any erosion experimentinvolving rills, it is important to characterize the flow (Polyakov & Nearing,
2003).
We calculated the nondimensional Reynold’s and Froude numbers for these experiments.Reynold’s
number is calculated as the ratio of flow velocity multiplied by hydraulic radius to the kinematic
viscosity of water. It is essentially a ratio of kinetic to viscous forces of the flow. Froude number is
the ratio of flow velocity to the square root of flow depth multiplied by the gravitational constant. It
represents a ratio of kinetic to gravitational flow forces.Depth of flow during target storms is
measured in the field directly. Manning’s roughness coefficientwas calculated for eachsegment of
rills using measuredvelocity; hydraulic radius (depth of flow) and
gradient.
4. Results and Discussion
Rainfall receivedduring 10:43 am to 2:13 pm on 31.08.2010 was monitored with a self recording rain
gauge at 15 minutes interval. Maximumintensity of 4.5 mm h-1 was recordedat 12:28 pm after 1:45
hrs of initiation of rain. A maximumof 5.6 Ls-1 discharge was collected from the entire catchment
after 30 minutes of peak rain.The velocity of runoff along the channels, monitored by dye tracer,
were linked with width-depth, and roughness coefficientsfor the present situation. Manning
coefficientis a good indicator of hydraulic roughness (Gilley et al., 1990; Nearing et al., 1997) and
analysis shows that this roughness tends to decrease flow velocity.Gradient and hydraulic radii are

4. considered as important factors of channel roughness and both are the result of distribution of
materials with in channel system. Flow velocity increases with decreasingmanning are roughness
value.
In the present study under the target storm, flow depth was not sufficient for bank full condition.
Although channelslope is consideredas an important factor of hydraulic roughness, flow velocity in
small rills are proved to be independedof gradient (Gimenez& Govers, 2001; Govers et al., 2007).
Slope does not influence the velocity of eroding rill because bed roughness increase with erosion,
counterbalancing the expectedeffectof slope on flow velocity (Figures 6, 7 and 8) (Gilley et al.,
1990).
In the present study, relations between gradient and channelrunoff velocity, and that between
width-depth ratio and velocity are found to be insignificant (Figure 5; Table 1). Manning’sroughness
coefficientsare proved to be important for predicting runoff velocities (Nearing et al., 1997), and for
constructing runoff hydrograph, lagtime calculation and analyzing runoff of upland areas (Poesen et
al., 2003). The Reynold’s number ranged between 300 and 61138 and Froude number varied from
0.02468 to 0.487169. Most of the data representsubcritical turbulent hydraulic regime (Figure 9).The
correlation betweenW/D ratio and flow velocity seemsto be insignificant in present condition. Bunte
and Poesen (1993) showed that on surface with roughness elements, localized turbulence remained
present and remained active in causing scour under the condition when Reynold’s number varies
above 250 (Nearing et al., 1997; 1999).
Sedimentconcentration was linked with runoff contributing area, but no significant relation was
established.
Sedimentsupply to the outlet depends mainly on the local factors like, discharge and localized slope
failure and erosion potentials. However,total runoff of eachrill has significant relation with runoff
contributing areas.
Visual observation of the sedimentin the rill suggests that the bedload moves in the shallow flow by
rolling along the bed, a process that, in either turbulent or laminar flow. In present analysis, by
associating Reynold’s Number against Froude Number, it is revealed that sub-critical turbulent flow is
dominant along the channels (i.e.rills).
Figure 1. The study area
Figure 2. Rill networks with nomenclature identity and Hierarchical Order of Gully Basin
Figure 3. Measurementof Width, Depth,Gradient and Surface Runoff in Gully Basin during study
period
Figure 4. Rainfall and resultant runoff from target storm (date-31.08.2010) of the entire gully basin
Figure 5. Relationship of flow velocity with (a) Manning roughness coefficient (n), (b) slope gradient,
(c) W/Dratio, and (d) correlation betweenManning roughness coefficient(n) and W/D ratio
Figure 6. Slope map of the study area Figure
Figure 7. Spatial distribution of flow velocity of the study area
Figure 8. Spatial distribution of roughness coefficient (manning) of the study area
Figure 9. Hydraulic flow regimes for the experimental data
5. Conclusions
Process based models for predicting runoff and erosion on upland areas require information on flow
hydraulics specially roughness coefficients.Dye tracing procedures were used in this study for
precise monitoring of flow hydraulics channels.The velocity and flow channels of dye cloud led to
some important understanding of processesacting in concernedarea. Sub-critical turbulent flow in

5. the channels is mainly controlled by hydraulic roughness.The ability to understand flow processes
will improve the cognition of process-response system operating in gully system of lateritic uplands.
Material transfer along the rills is not directly controlled by the slope, but by the hydraulic roughness,
where gradient may be a factor but not the sole determinant. For predicting flow velocity, roughness
coefficient(Manning) and material transfer had to be givenmore importance over gradient and
other hydraulic variables of channels. The accurate monitoring and prediction of hydraulic roughness
coefficientswill improve our ability to understand and properly model upland flow hydraulics.
References
Abrahams, A. D.,& Parsons, A. J. (1994). Hydraulics of interrill overland flow on stone-covereddesert
surface.
Catena,23, 111-140. http://dx.doi.org/10.1016/0341-8162(94)90057-4
Abrahams, A. D.,Parsons, A.J., & Luk, S. H.(1986). Field measurement of the velocity of overland
flow using dye tracing. Earth Surf.Process. Landforms,11,653-57.
http://dx.doi.org/10.1002/esp.3290110608
Abrahams, A.,& Li, G. (1996). Rill Hydraulics on a semiarid hillslope, Southern Arizona. Earth Surface
Processesand Landforms,21, 35-47.http://dx.doi.org/10.1002/(SICI)1096-9837(199601)21:1<35::AID-
ESP539>3.0.CO;2-T
Bryan, R.B., Govers, G.,& Poesen,J. (1989). The conceptof soil erodibility and some problems of
assessment and application. Catena,16, 393-412. http://dx.doi.org/10.1016/0341-8162(89)90023-4
Bunte,K.,& Poesen,J. (1993). Effects of rock fragment covers on erosion and transport of
noncohesive sediment by shallow overland flow. WaterResour.Res.,29, 1415-1424.
http://dx.doi.org/10.1029/92WR02706
Chow, V. T. (1959). Open-channelhydraulics.New York: McGraw-Hill. www.ccsenet.org/jggJournal of
Geography and Geology Vol. 4,No. 4; 201210
Favis-Mortlock, D.T.,Boardman, J.,Parsons, A. J., & Lascelles, B.(2000). Emergence and erosion: a
model for rill initiation and development. Hydrol.Process.,14, 2173-2205.
http://dx.doi.org/10.1002/1099-1085(20000815/30)14:11/12<2173::AID-HYP61>3.0.CO;2-6
Finkner, S.C., & Gilley, J. E. (1988). Measurementof flow components on upland areas using dye
dilution techniques. Transactionsofthe ASAE,31(4), 1086-1091.
Foster, G. R., Huggins,L. F., & Meyer,L. D.(1984). A laboratory study of rill hydraulics: I. Velocity
relationships. TransactionsofASAE, 27, 790-796.
Gilley, J. E.,Kottwitz, E. R.,& Simanton, J. R. (1990). Hydraulic characteristics of rills. Trans.ASAE,
33,1900-1906.
Gilley, J. E.,Kottwitz, E. R.,& Wieman, G. A. (1992). Dracy-Weishachroughness coefficientsfor gravel
and cobble surfaces. J. Irrig.Drain. Eng.,118,104-112.
http://dx.doi.org/10.1061/(ASCE)0733-9437(1992)118:1(104)
Gimenez,G., & Govers, G. (2001). Interaction betweenbed roughness and flow hydraulics in eroding
rills. WaterResources Research, 33(2),349-358.
Giménez,G., & Govers, G. (2002). Flow detachment by concentrated flow on smooth and irregular
beds. Soil Science Society ofAmerica Journal,66(5), 1475-1483.http://dx.doi.org/10.2136/sssaj2002.1475
Govers, G. (1991). Rill erosion on arable land in central Belgium:rates, controls and predictability.
Catena,18,133-155. http://dx.doi.org/10.1016/0341-8162(91)90013-N
Govers, G. (1992). Relation betweendischarge, velocity and flow area for rills eroding loose, non-
layered materials. Earth Surface Processesand Landforms,17,515-528.
http://dx.doi.org/10.1002/esp.3290170510

6. Govers, G., Giménez,R.,& Oost, KVan. (2007). Rill erosion: exploring the relationship between
experiments,modellingand field observations. Earth Science Reviews, 84(3-4), 87-102.
http://dx.doi.org/10.1016/j.earscirev.2007.06.001
Jehangir, M.,& Zokaib, S.(2000). Annual progress report – PARDYP(The People and Resource
Dynamics in Mountain Watersheds of the HKH),Pakistan, International Centre for Integrated
Mountain Development,Katmandu,Nepal.Lei, T.,Nearing, M.A., Haghighi,K., & Bralts, V. F. (1998).
Rill erosion and morphological evolution: a simulation model. WaterResour. Res.,34, 3157-3168.
http://dx.doi.org/10.1029/98WR02162
Mancilla, G. A. (2001). Prediction of rill density, transport capacity and associated soil loss of different
tillage systems under winter conditions. M.S.thesis, College of Engineering and Architecture,
Washington State University.
Mancilla, G. A., Chen,S.,& McCool,D. K.(2005). Rill density prediction and flow velocity distribution
on agricultural areas in the Pacific Northwest. Soil andTillage Research, 84, 54-66.
http://dx.doi.org/10.1016/j.still.2004.10.002
Nachtergaele,J., Poesen,J., Sidorchuk,A., & Torri, D. (2002). Prediction of concentrated flow width
in ephemeral gully channels. Hydrological Processes,16,1935-1953. http://dx.doi.org/10.1002/hyp.392
Nearing, M.A., Norton, L. D.,Bulgakov, D.A., Larionov, G. A.,West, L. T.,& Dontsova, K.M.
(1997).Hydraulics and erosion in eroding rills. WaterResources Research, 33,865-876.
http://dx.doi.org/10.1029/97WR00013
Nearing, M.A., Simanton, J. R.,Norton, L. D.,Bulygin, S.J., & Stone,J. (1999). Soil erosion by surface
waterflow on a stony, semi ar id hillslope. EarthSurface Processes andLandforms,24(8), 677-686.
http://dx.doi.org/10.1002/(SICI)1096-9837(199908)24:8<677::AID-ESP981>3.0.CO;2-1
Obiechefu,G. C.,& Morgan, R.P. (1994). Rill initiation and evolution: a precursor for understanding
gully erosion. American Society ofAgricultural Engineers,94, 2584-2599.
Poesen,J., Nachtergaele,J., Verstraeten, G.,& Valentina, C. (2003). Gully erosion and environmental
change:importance and researchneeds. Catena, 50,91-133. http://dx.doi.org/10.1016/S0341-
8162(02)00143-1
Polyakov, V. O., & Nearing, M.A. (2003). Sediment transport in rill flow under deposition and
detachment conditions. Catena,51, 33-43. http://dx.doi.org/10.1016/S0341-8162(02)00090-5
www.ccsenet.org/jggJournal of Geography and Geology Vol. 4, No. 4; 2012